After nine years of battling with difficult ground, extensive archaeology and even the effects of earthquakes, the new Athens Metro eventually opened early last year. A final, particularly difficult, section was completed in October and opened in November. Athenians seem pleased with the marble-floored stations and the relief the two new lines have brought to the congested city.
They should be even more pleased by 2004 when the Olympics is to be held in Greece. An estimated Dr176bn ($440M) programme of four extensions to the two new lines is expected to be completed in time to facilitate travel by visitors to the event.
One small section of Line Three remains. This is the difficult Monastiraki station, situated underneath the famous Acropolis, the most ancient part of the city and with the greatest archaeological obstacles. “Archaeology is deeper here, up to 7m, and more than 3,000 years old,” says Stelios Koukoutas, site manager for all tunnels and underground works at client Attiko Metro. Ground is also extremely difficult, typically a wet schist said to be like mud at times.
History
The project has battled with these factors since 1991 (T&TI November 1996). The turnkey contract with the 19-firm Olympic Metro international consortium completed its 17.6km of tunnelled system a little over a year beyond the original schedule.
Stations were built mainly by cut and cover in the suburban sections of the city, though six in the city centre used NATM methods to create their 110m long underground spaces and side platforms.
Twin track running tunnels were constructed with cut and cover for 6.5km of the route and with NATM for a few short and difficult sections. NATM was also used for transition and three track sections, side tunnels and other underground structures.
TBMs
For some 11.7km length of route two, 9.5m od TBMs named Jason and Persephone were used to bore twin track running tunnels. These machines, built by NFM in France and employing Mitsubishi technology from Japan, were of a hard rock configuration using 63 cutter discs and 200 drag picks.
The TBMs erected an 8.48m id grouted concrete segment ring. They worked at depths of 10m to 20m, measured to the final tunnel crown, to avoid ancient relics found down to around 5m depth in most areas.
Although they had made good progress in many sections of the ground, they also ran into a large number of problems, particularly with overbreak voids forming above the cutter head.
Because of this, the particularly difficult final section of tunnel to Dafni, just under 1km long, was excavated using an open face shield from Herrenknecht.
“The voids were one of the most critical problems in the whole project,” says Koukoutas. In many cases the ground collapsed, with propagation of the soil failure forwards and above the head or backwards.
Many incidents were not extensive and resulted in only small surface settlement. But there were as many as 450 incidents of overbreak of more than 1m by 500mm by 2m, with in some cases a ‘ravelling’ of the ground and formation of larger caverns, “usually conical in shape and several metres high”. These were grouted.
In the most extreme cases, overbreak extended to the surface. Two incidents saw loss of a street kiosk on Panepistimiou Street and a hole through in the National Garden. Fortunately no incidents were under buildings, and there were no fatalities or major injury.
Geology
Problems stemmed from the extremely difficult and variable ground of Athens, sometimes disturbed by ancient wells or unknown structures. Athens sits on a series of schist formations, an Upper Cretaceous flysch-like sediment. It includes clayey and calcerous limestones, greywackes, siltstones and shales, limestones and marls. Low degree metamorphosis has formed metasandstones and metasiltones, and local igneous activity has created periodotitic and dibasic bodies.
All this was then subject to intense folding and thrusting during the Eocene followed by extensive faulting. In addition, there has been weathering and alteration of the deposits. The resulting rock is highly heterogeneous and anisotropic. The machines frequently met areas where the ground was loose and fine grained, soil-like or in the form of slipping flakes from tectonically disturbed phyllite, a weak black shale.
The TBMs were designed to deal with various mixes of rock and soil, but they could not always handle these conditions without the collapse of surrounding material. A particularly awkward condition was where a block of hard rock, such as sandstone, was encountered in weak ground. The ‘floating’ block tended to be ripped or dragged from position, extending the cut line.
Though the schist is relatively impermeable, it weakens when saturated with water. It was particularly problematic around damaged sewers or water pipes and when ancient wells were discovered. These sometimes dropped as deep as 20m and were full of mud and water. They were additionally difficult because the ground around them was disturbed and weakened and they had potential archaeological value.
Wells
At the end of the Line Three drive at Syntagma Square, Athens’ central area, a short overrun was abandoned because a great number of ancient wells were encountered. The final short section was done with NATM and the machine was dismantled, with its shield left in the ground. NATM work to connect to the Monastiraki station continues.
Wet weakened ground near Dafni led the Attiko Metro to purchase an open face shield for the drive at the southern end of Line Two. The alignment runs past one of Athens’ hills which traps water in winter.
Previously difficult ground was treated, either from the TBM or a pilot tunnel driven ahead, typically a 3m diameter NATM drive 1.5m below crown. Glass-fibre spiles, grouting and void filling were used and the shotcrete support for the pilot became a stabilising factor for the main drive.
In some cases, treatment was carried out from the surface, in particular along a 200m section near the Plaka, close to Acropolis. Working in a closed off street, jet grouting rigs formed a pattern of columns in the ground to stabilise ahead of the machine.
Open face
“But for the Dafni to Ag Ioanni section we were driving under buildings. This ruled out surface treatment,” says Koukoutas. The contractor proposed an open shield to make up time. The client ordered a Dr4bn ($10M) shield, nicknamed the Dafni MetroMouse, with a 180° section covered by seven hydraulically operated forepoling blades. Excavation is by a 1,300mm diameter AC Eickhoff boom cutter with two telescopic loading shovels beneath.
A screw conveyor was used for most mucking out, passing back through a crusher onto a primary conveyor for loading a Mühlhäuser truck and Schöma locomotive mucking out system.
The drive began in November 1998 and finished well in March 1999, Koukoutas says. Settlements were kept to around 25mm maximum.
Extensions
The shield is now being dismantled for transport to Sepolia, the first of four new sections of tunnelling. A Dr12.5bn ($32.6M) design/build contract was let last July to a joint venture of Austria’s Alpinebau, Germany’s Wayss und Freitag and Greek firm Attikat for 3km of tunnel running north westwards to Thivon from the depot at Sepolia.
Work has begun on a 500m long NATM section under the Kifisios river just beyond the depot. This will carry three lines, one into the depot. The main 2.5km section will begin from the other end to join this ‘trumpet’. One station at Ag Antonios has been let as a Dr3.2bn ($8.35M) contract to Thermo.
A second extension is being bid (T&TI last month). This runs 5km from Ethiki Amyna to Stavros and is crucial for the Olympics in 2004. At Stavros, the line will intersect a new Athens ring highway to connect the new airport at Spata.
An EPB TBM is the most likely means to make 4.5km of the section, but a decision awaits award of the design/build contract next month. A small section of cut and cover work is included with NATM proposed for a three-track section. Only two of the four stations will be constructed before 2004.
A more complex extension is being prequalified. This is for the continuation of Line Three from Monastiraki to Egaleo, providing another three stations. The route has been realigned, with two single tunnels swerving around the previously planned Keramikos station. At the same time Monastiraki was removed from the main contract.
Eight contractors are competing for the estimated Dr46bn contract to be done either by TBM or NATM.
Another Dr45bn ($117M) extension will take Line Two south from Dafni to a new station at Ilioupoli, a distance of around 1km to be done by NATM.
Related Files
Monastiraki station
